Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Semiconductor devices are now being fabricated with features less than a quarter of a micron across.
As merely an example, etching processes are often used to remove or partially remove a layer to form structures there from. Etching is often performed by an etching tool, such as a dry etcher or wet etcher. The wet etcher often includes a vessel that has an etchant chemical to selectively remove one material from another material. The dry etcher often includes a plasma source and treatment chamber. The dry etcher often uses gases such as fluorine bearing species and chlorine bearing species to remove semiconductor materials such as silicon, or metal such as aluminum, or dielectric material such as silicon oxide.
Much work has been done to use real-time metrology to characterize semiconductor manufacturing processes and the effect of these processes on the wafers being processed. In contrast to ex situ metrology, which allows detailed scrutiny of the wafer surface, real-time metrology requires in situ measurement, which rarely allows such a close investigation of the wafer. Consequently, one needs to measure parameters such as the power being delivered into a process chamber, or the gases inside a process chamber in order to make inferences about the state of the wafer.
Typical objectives of real-time metrology for semiconductor processes include identification of a particular wafer state, such as that point at which a particular thin film is fully etched in a plasma etch process (the end point); or characterization of key process parameters, such as the rate at which a thin film is being deposited or etched.
One approach to measuring a gas inside a process chamber is to use a spectrometer to measure the light emitted from the plasma inside the process chamber. Another approach to measuring the gas inside a process chamber is to use a system comprising a self-contained plasma chamber with a spectrometer to measure the light emitted from the self-contained plasma. For example, in using such a system, the self contained plasma chamber of the detection apparatus would be in fluid communication with the processing chamber, such that the gas from the processing chamber can flow and/or diffuse into the plasma chamber of the detector apparatus.
In both of these approaches, when the gas is excited by the self-contained plasma, a fraction of the gaseous particles, which can include atoms, molecules, and molecular fragments, will have one or more of their electrons excited to a higher-energy state. When these electrons fall back to their lower-energy states, photons, with energy equal to the energy lost by the electrons, are emitted from the gaseous particles. The energy of each of the photons is characteristic of the particle (atom, molecule, or molecular fragment) from which it was emitted.
Since the photon energy is characteristic of the gaseous particle from which it was emitted, and there is a one-to-one relationship between photon energy and wavelength (or frequency, which is inversely proportional to wavelength), measurement of the intensity of the emitted light as a function of wavelength can provide information on the gaseous particles present in the plasma, thus providing information on the chemical composition of the gas.
For example, FIG. 1 shows a conventional emission spectrum taken from a chamber having a plasma comprising air. The y-axis of FIG. 1 indicates the intensity of the emission (in arbitrary units), which generally increases with increasing concentration of the emitting particle in the gas mixture. The x-axis of FIG. 1 indicates the wavelength of the emission, measured in nanometers.
The spectrum of FIG. 1 is characteristic of nitrogen gas in molecular form, which is to be expected given that air comprises approximately 80% such molecular nitrogen. Apart from revealing the presence of molecular nitrogen, however, the emission spectrum of FIG. 1 provides relatively little information
Specifically, in addition to nitrogen, air also contains approximately 20% oxygen. However, the spectrum of FIG. 1 lacks any meaningful indication of the presence of the oxygen.
This is because gas mixtures will typically contain many different molecules and/or atoms. Gases in molecular form in general produce spectra which consist of bands corresponding to electronic transitions, which are comprised of sub bands corresponding to transitions between vibrational states, and these sub-bands themselves comprise many individual lines corresponding to transitions between different rotational states. The finite resolution of the spectrometer blurs these many lines together into continuous bands. Although the spectrum for molecular nitrogen exhibits more bands than most molecules or molecular fragments, a typical spectrum of most gases in molecular form is still usually crowded with bands, which in general overlap from one gas component to another. This makes it difficult to ascertain the true chemical composition of the gas mixture utilizing conventional spectroscopic techniques when the gas mixture is dominated by gases in molecular form. By contrast, the emission spectrum of atoms tends to consist of isolated lines, many of which are sufficiently separated in wavelength that they can be resolved by conventional spectrometers.
Still another difficulty with the use of conventional spectroscopic techniques lies in the difficulty of associating the peaks of a spectrum to the particular molecules or molecular fragments. For example, the documentation of emission spectra from atoms is very detailed and comprehensive. An excellent source of information on the specific wavelengths that a particular atom emits, along with the relative intensity of the emission at each of the specific wavelengths, is available from the National Institute of Standards and Technology (NIST) at http://physics.nist.gov/PhysRefData/ASD/lines_form.html.
By contrast, the emission spectra from molecules or molecular fragments are much less well documented. Thus even if the emissions of a particular molecule or molecular fragment could be segregated from others in a spectrum, correlation of this information to a known component of the gas mixture would not be possible in many instances. The problem is exacerbated by the highly energetic state of the plasma, in which unstable molecules and molecular fragments can form and then dissociate in very short time scales.
Previous work has been carried out on atomic emission detectors for analytical techniques such as chromatography. Frequently, such detectors are used to vaporize and analyze liquids, often with large amounts of argon or another carrier gas added to the mixture. Although these approaches allow detection of atomic emission, they are designed to operate at atmospheric pressure, and are ill-suited for analysis of the gaseous environments inside of process chambers such as those used in the processing of semiconductor devices.
Another related analytical technique is Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). This technique uses an ICP torch comprising concentrically arranged quartz tubes with a water-cooled RF coil. In conjunction with an argon carrier gas, the ICP torch creates a high temperature plasma (approximately 7,000 K) that atomizes and excites the material to be analyzed. ICP-OES is frequently used for the analysis of trace metals. Because of the torch-like configuration, the high flow rate of the argon carrier gas, and the operation at atmospheric pressure, however, this technique is also unsuitable for analysis of the environments arising during the fabrication of semiconductor devices.
A final difficulty with the use of conventional spectroscopic techniques is in the strength of the signals that are to be observed. Specifically, as device size shrinks, the area of the material that is changed by the process being studied may represent only a very small fraction of the overall area. For example, in a typical step of etching a via with a plasma, the area of dielectric layer to be removed is only about 1% or less of the total area. Since conventional measurement techniques such as optical emission spectrometry (OES) or radio frequency (RF) measurements measure an overall plasma state, the effect from other interferences can overwhelm the signal of the measurement.
Silicon-containing dielectric films (such as silicon nitride, silicon dioxide, doped silicon dioxide, and low-k films) are commonly present in semiconductor devices. These silicon-containing dielectric films are first deposited as a blanket layer, and then removed in selected regions exposed by photolithography. Removal of the dielectric layer is accomplished by etching, which must be performed with high precision so as not to damage the material underlying the dielectric. The key to precise etching is to accurately determine the process endpoint—the point in time at which the silicon dioxide is no longer being removed.
Typically, however, the exposed area of the dielectric films being etched will be very small (for example a few percent or less of the total area), resulting in any signal associated with the endpoint of such etching being very small. The combination of (i) a small signal, (ii) obscuring of signals by multiple bands overlapping one another, and (iii) the difficulty of identifying the gas species associated with any particular emission, conventionally renders detection of the endpoint of the etching process very difficult.
From the above, it is seen that improved techniques, systems, and methods for analyzing gas mixtures and determining process endpoint, are desired